The field of the invention is nuclear magnetic resonance (MR) methods and systems. More particularly, the invention relates to the measurement of T.sub.2 in vascular blood and the use of that measurement to determine coronary flow reserve.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, and this signal may be received and processed to form an image or to measure characteristics of the excited spins.
One characteristic of the spins that can be measured using MR is the T.sub.2 constant. When the B.sub.1 excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.t. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of M.sub.t.
The amplitude A of the emission signal (in simple systems) decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-1/T.sub.2
The decay constant 1/T.sub.2 is a characteristic of the process and it provides valuable information about the substance under study. The time constant T.sub.2 is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant, and it measures the rate at which the aligned precession of the nuclei dephase after removal of the excitation signal B.sub.1.
It is well known that the T.sub.2 time constant of blood is determined in part by the degree to which the hemoglobin is oxygenated. As reported by G. A. Wright et al., "Estimating Oxygen Saturation of Blood in Vivo with MR Imaging at 1.5 T", JMRI 1991; 1:275-283, the percentage of hemoglobin that is oxygenated (% O.sub.2) can be quantitatively measured using a series of T.sub.2 -weighted MR images. This MR oximetry method works well when measuring blood oxygenation in large vessels that have little motion and that are positioned to produce high SNR images. The MR oximetry method does not work well, however, on small centrally located vessels, such as the coronary veins that drains the heart and vessels that have considerable motion.
Coronary flow reserve is defined as the ratio between the peak and basal coronary blood flows at the same perfusion pressure (Q.sub.peak /Q.sub.basal) The coronary flow reserve is a characteristic measurement of global coronary function, and it is decreased in people with significant coronary artery disease. Measurement of the coronary flow reserve provides valuable diagnostic information to the physician.
Quantitative and non-invasive methods to assess coronary flow reserve do not exist at present. Current methods rely on the imaging of pharmaceuticals to visualize qualitative changes in myocardial perfusion. Direct measurements of coronary flow reserve using Doppler ultrasound and MR methods are difficult to perform due to technical challenges and the complexities of the in vivo environment. The development of a flow reserve measurement which is both quantitative and non-invasive could significantly impact patient management and treatment planning for ischemic heart disease.